This invention relates to an arrangement for the parallel tensioning of printing plates on a plate cylinder.
More specifically, the invention relates to a tensioning arrangement for use in a printing machine of the type in which the plate cylinder is disposed in a cylinder gap having a front wall. In an arrangement of this type, the ends of the printing plates are adapted to be clamped to a divided front tensioning bar and to a divided rear tensioning bar, the bars being located in the cylinder gap and being movable substantially in the circumferential direction of the cylinder in order to tension the plates.
Axially spaced front tensioning screws are connected to the front bar and bear directly against the front wall of the gap. Adjustment of the front screws results in substantially circumferential movement of the front tensioning bar.
Means are provided for effecting quick-action tensioning of the printing plates by moving the rear tensioning bar circumferentially. These means comprise a quick-acting mechanism for initial tensioning and further comprise rear tensioning screws for final tensioning. The quick-acting mechanism comprises a pressure strip extending axially along the cylinder gap. The pressure strip is formed with recesses having inclined surfaces which coact with axially fixed members having wedge surfaces. A central actuator is adapted to move the pressure strip axially back and forth and, when the strip is moved axially in one direction, the wedge surfaces act against the inclined surfaces of the recesses to move the rear tensioning bar circumferentially in one direction for initial tensioning of the printing plates.
The rear tensioning screws are connected to the rear tensioning bar and bear against the pressure strip. Adjustment of the rear screws results in substantially circumferential movement of the rear bar to effect final tensioning of the printing plates.
An arrangement of this kind is disclosed in West German Patent Specification 3,516,682 and serves to enable printing plates to be tensioned in parallel with a defined tensioning force. This achieves accurate register and, in the final tensioning, the known facilities for correcting the print employing a divided tensioning bar and associated tensioning screws may be retained. It is difficult to manufacture and assemble the constructional components with such accuracy that the rear divided tensioning bar is moved exactly parallel to the front gap wall. Overloading of the printing plate is quite possible. Automatic retensioning during machine operation is impossible.